Sodium trimetaphosphate (also STMP), with formula Na3P3O9, is a metaphosphate of sodium. It has the empirical formula NaPO3. It is the sodium salt of trimetaphosphoric acid. It is a colourless solid that finds specialised applications in food and construction industries. STMP is prepared by heating sodium polyphosphate at a few hundred degrees. [2]

Although drawn with a particular resonance structure, the trianion has D3dsymmetry.